Resins for extrusion coating on paper, board, aluminum, etc., are designed with broad molecular weight distribution (MWD) and low extractables. In extrusion coating application the polymer is processed at high temperature conditions, typically 280° C. to 350° C. Broad MWD (which typically requires a significant high molecular weight fraction) is necessary for good processability during coating (neck-in and draw down balance), while low extractables are needed for low smoke formation during coating, at high temperature conditions, and/or for food contact compliance.
Broad MWD low density polyethylene (LDPE) is made up of low and high molecular weight polymer molecules, and an average molecular weight will determine the melt index. The extractable fraction increases with an increasing fraction of low molecular weight molecules, and is enhanced by increasing short chain branching frequency at low molecular weight molecules. In view of this combination of features, there is typically a trade-off between coating performance and extractable level.
Typically LDPE resins with broad MWD are made in an autoclave reactor or a combination of autoclave and tube reactors. Broad MWD resins can be achieved in autoclave reactor systems by promoting long chain branching and through the inherent residence time distribution by which molecules will undergo shorter (low molecular weight) or longer (high molecular weight) growth paths.
WO 2013/083285 teaches, among other things, a LDPE having an Mw/Mn which is greater than 15, a storage modulus G′ (5 kPa) which is above 3000, and a vinylidene content which is at least 15/100k C, compositions and a process for producing the LDPE in a tubular reactor by radical initiated polymerization where the polymerization is performed by reacting the ethylene monomer under the action of one or more radical initiators, e.g., peroxides, wherein the amount of used radical initiator is at least three times the conventionally used amount. The storage modulus G′ at loss modulus G″=5 kPa is shown to be generally higher for the inventive LDPE than the standard tubular LDPE produced with conventional techniques.
WO 2013/078018 A2 and WO 2013/078224 teach that tubular reactor products, which are suitable for extrusion coating application by having broad MWD, low extractables and high enough melt strength and rheological G′, can be made without any chemical modification, for instance without the use of cross-linking agents in reactors, separators, extruders, etc.
The intrinsic drawback of a more uniform residence time distribution in the tubular versus the autoclave process, which negatively limits the broadness of the MWD, is compensated by a careful selection of process conditions, like reactor configuration, peak temperature, reactor inlet pressure, conversion level, fresh ethylene and/or CTA distribution, etc.
For the resins described in the above patents it has been found that at a given melt index (I2), the melt strength and rheological G′ can be increased at the cost of extractable level by synthesis of products at higher absolute (abs) Mw and broader MWD by adapting the process conditions.
Schmidt et al (Macromolecular Materials and Engineering, Vol 290, p 4004-414, 2005) describe and model the impact of segmented flow distribution in a tubular reactor and its effect on formation of an ultra-high molecular weight tail in MWD. Flow segmentation will always be present to some extent by the laminar boundary layers at the wall, even when a highly turbulent flow regime is maintained. Flow segmentation in a tubular reactor can be enhanced by dynamic and or static fouling. The conditions required for making tubular extrusion coating resins lead to branched, high molecular weight polymer susceptible to fouling and or chain entanglement with polymer already adhered at the inside tubular wall. The polymers cited in WO 2013/078018 A2 and WO 2013/078224 have been made at minimum flow segmentation conditions as demonstrated by the low level of the ultra-high molecular weight tail in the light scattering gel permeation chromatography (LS GPC) curve. However it has been found that depending on the train configuration and/or operation condition this ultra-high molecular weight tail can be significantly increased as shown by LS absolute GPC data. The presence of this ultra-high molecular weight tail will significantly increase Mw(abs) and broaden the MWD. However surprisingly it has been found that this increase in Mw(abs) has minor impact on the MS and G′ performance of the produced material, leading to a higher Mw(abs) and broader MWD design for the same MS and G′ performance. This broader MWD design at a fixed melt index implies more ultra-high molecular weight as well as more low molecular weight. This typically leads to a higher extractable level.
Thus, there is a need for new ethylene-based polymers with low extractables even when the MWD is broadened as in the case of enhanced flow segmentation in a tubular process (as analyzed by LS GPC). These new polymers are suitable for extrusion coating applications, and can be made in a tubular process showing an increased tendency to flow segmentation. There is a further need for such polymers that can be prepared without any chemical modification, for instance without the use of cross-linking agents in reactors, separators, extruders, etc., or the use of blending operations.